1. Field of the Invention
The present invention relates to solar cells for converting incident light into electricity and, more specifically, to vertical junction solar cells and to an improved structure thereof which reduces radiation degradation within the vertical junction solar cell.
2. Description of the Related Art
In a solar cell, light is absorbed at various depths from the top surface of the cell depending on the wavelength of the light impinging the cell. The impinging light needs to be absorbed within one diffusion length of the P-N junction. If the impinging light is not absorbed within one diffusion length of the P-N junction, the electron hole pair created by the absorption of the photon will merely recombine and release its energy as heat instead of contributing to the cell's electrical output. The term. "diffusion length" as used herein describes the mobility of a charge carrier which is generated by absorption of optical energy, i.e., incident light.
By way of example, the P-N junction in a typical solar cell is only 0.3 .mu.m from the front surface of the cell, which for present purposes places the P-N junction virtually at the front surface. Thus, assuming that all of the light impinging the solar cell is absorbed within the first 75 .mu.m below the front surface of the cell, so long as the diffusion length is 75 .mu.m or more the cell can convert all of the electron hole pairs created by the absorption into electricity.
Radiation adversely effects the cell by damaging its crystal-line orderliness, resulting in a shortening of the distance that the electron-hole pair can travel, i.e., shortening the diffusion length. At a dosage of 1.times.10.sup.16 MeV, the diffusion length of a 10 ohm-cm cell may be reduced to something on the order of 10 .mu.m. Thus, the original cell with a useful collection region to a depth of 75 .mu.m would have its collection region decreased to only about 10 .mu.m, thus limiting the power output to that produced by light which is absorbed in the top 10 .mu.m of the cell.
One way of attempting to alleviate the problem of radiation damage shortening the diffusion length of the cell is to bring the P-N junction closer to the region where the light is absorbed. This can be accomplished simply by placing the P-N junction deeper below the top surface of the cell. However, this solution creates a disadvantage because the response, i.e., output, of the cell to shorter wavelength light which is absorbed very close to the cell surface is lost.
Another approach to alleviating the problem of radiation effectively shortening the available diffusion length in a cell is the vertical junction cell approach. Vertical junction solar cells are formed by etching a plurality of parallel grooves in the top surface of the cell. The grooves define a corresponding plurality of walls each having a top and side surfaces. Typically, the grooves are etched to a depth of approximately 75-100 .mu.m. With such a structure, light entering the cell at the top surface of the walls will most likely be entirely absorbed within the cell since the P-N junctions extend along the side surfaces of the walls as well as along the top surface of each wall, and these P-N junctions are within one diffusion length of the absorbed light even at reduced diffusion lengths so long as the distance between grooves, i.e., the wall width, is selected properly. However, light entering the grooves and impinging the cell at the bottom of the grooves is effectively the same as light which is absorbed in a planar cell at a depth well beyond the reduced diffusion length of a radiation damaged cell. Thus, most of the light entering the groove bottom is not absorbed in the cell within one diffusion length of a P-N junction and, therefore, does not contribute to the cell output.
Certain efforts to improve the efficiency of silicon solar cells have centered around two approaches. The first is the use of low resistivity silicon with a passivated front surface to significantly increase cell output voltage. The second is the use of high lifetime, textured cells with a back-surface-field (BSF). However, neither of these approaches are particularly effective where the solar cell is to be used in outer space because the efficiency improvements are negated by more severe radiation degradation encountered in the space environment. Specifically, low resistivity silicon exhibits a greater decrease in bulk diffusion length when subjected to particulate irradiation than higher resistivity silicon.
For higher resistivity silicon a BSF is necessary to achieve high efficiencies. However, BSF cells degrade more rapidly under irradiation. The use of a thinner solar cell may alleviate the degradation problem, but also results in reduced optical absorption. Thus, neither of these two approaches to increasing efficiency of the cells is well-adapted to high radiation environments such as space satellites.